Northeastern Section - 54th Annual Meeting - 2019

Paper No. 12-1
Presentation Time: 1:30 PM


GILOTTI, Jane A., Departmernt of Earth and Environmental Sciences, University of Iowa, Iowa City, IA 52242, MCCLELLAND, Bill, Department of Earth and Environmental Sciences, University of Iowa, Iowa City, IA 52242 and COBLE, Matthew, Geological Sciences Department, Stanford–USGS Ion Microprobe Laboratory, Stanford University, CA 94305

Quartzofeldspathic gneiss that hosts eclogite is the dominant lithology of the continental ultrahigh-pressure (UHP) terrane in the North-East Greenland Caledonides. Both lithologies contain coesite, indicating all the rocks reached mantle depths. The best-preserved gneiss is a clinopyroxene-bearing granulite with HP phases (kyanite, phengite, omphacite and rutile) included in garnet. Titanite is common in the retrogressed gneiss and shows a textural progression from overgrowths on matrix rutile to polycrystalline grains with no rutile to euhedral porphyroblasts. Titanite from 6 gneisses has been analyzed for U-Pb and trace elements by SHRIMP-RG using McClure Mountain titanite as a standard. Weighted mean 206Pb/238U dates, using data-defined common Pb values, range from 347 ± 6 to 321 ± 7 Ma. Regression of data from all samples defines an intercept of 331 ± 4 Ma (MSWD=2.6), which is taken as the age of rutile-to-titanite conversion. Four samples show concave-down trace element patterns, similar to igneous titanite, interpreted as formation in the presence of a melt; indeed, three of the samples contain thin, wispy leucosomes. Large (>600 µm), zoned subhedral titanite from a metagranitoid gneiss preserves three distinct trace element patterns that can be related to melting, metamorphism and garnet breakdown. Zr-in-titanite temperatures, calculated at 0.5 GPa and aSiO2=aTiO2=1, range from 690–723°C. The preservation of different trace element and age domains argues against simple cooling ages.

UHP ages from coesite-bearing zircon in eclogite and gneiss are ≈365–350 Ma, or approximately 35–20 m.y. earlier than titanite growth. Titanite from UHP gneiss in NE Greenland formed over a prolonged exhumation history also recorded by zircon: (1) Phengite decompression melting at the quartz-coesite transition starting at ≈347 Ma; (2) Granulite to amphibolite facies metamorphism accompanied by pegmatite emplacement between 350 and 340 Ma; and (3) Continued amphibolite facies exhumation with titanite growth through 320 Ma. This contrasts with many UHP terranes where titanite grows within < 4–15 million years of peak P. Slow exhumation agrees with a tectonic setting at the base of an overthickened, overriding plate where UHP occurs in intracontinental subduction zones, as has been proposed for present day Tibet.